Apparatus for collecting particles

ABSTRACT

Detail is apparatus for collecting particles from a liquid, including a first electrode having an upper surface defining one or more substantially circular areas of a predetermined diameter separated from a counter electrode, having an uppersurface bounding at least in part the circular area or areas of the first electrode, by a predetermined gap and equipment for applying an alternating potential difference of a predetermined magnitude at a predetermined frequency lower than the charge relaxation frequency of the liquid across the electrodes whereby particles from the liquid are focused onto the circular area or areas of the first elect through an induced electro-osmotic flow in the liquid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of International ApplicationNo. PCT/GB2004/000533 filed on February 12, 2004 and published inEnglish on August 26, 2004 as International Publication No. WO 2004/071668 Al which application claims priority to Great Britain ApplicationNo. 0303305.7 filed on February 12, 2003, the contents of which areincorporated by reference herein.

FIELD OF THE INVENTION

The present invention is generally concerned with apparatus forcollecting particles. The present invention is particularly, althoughnot exclusively, directed to surface bound sensors incorporating planarelectrode configurations capable of collecting or focusing particlesfrom a liquid on an upper surface thereof.

BACKGROUND OF THE INVENTION

The optical detection of particles, such as bacteria and viruses,adhered to a sensing medium by monitoring changes in refractive index ofthe medium and/or light scattered or emitted from the particles is awell-known technique. See, for example, International Patent applicationWO 01/42768 and references therein.

However, the sensitivity of the technique is limited by slow diffusionof the particles in the liquid to the sensor surface, even with stirringor agitation. Consequently, there is a need to improve the concentrationof particles on the sensing medium of sensor surfaces.

It is well-known that a particle can become polarised in aninhomogeneous electric field and that the interaction of the induceddipole with the field leads to the movement of the particle towards oraway from the area of field inhomogeneity. These effects, termedpositive or negative dielectrophoresis respectively, depend on theproperties of the particle and the liquid. Dielectrophoresis forms thebasis for the separation of particles in a large number of apparatus.

However, one problem with the application of the technique so as toenhance particle deposition to a sensing surface, is that the areas ofgreatest electric field inhomogeneity are at the electrode edges.Consequently, particles accumulate at the edges and not on the surfaceof the planar electrodes.

A third form of dielectrophoresis can be used to separate submicrometreparticles between planar electrodes (N. G. Green and H. Morgan, J. Phys.D, Appl. Phys., 1998, 31, L25-L30). The technique, known as “abnormaldielectrophoresis” because particles can collect on the surface of theelectrodes, relies on an induced electro-osmotic flow in the bulk liquidwhen an alternating potential difference is applied to the electrodes.The bulk flow, which is driven by an interaction between the oscillatingelectric field and the diffuse double layer of charge on the electrodes,originates in the gap between the electrodes, and progresses over thesurface of each electrode. For a detailed explanation, see N. G. Greenet al., J. Colloid Interface Science, 1999, 217, 420-422 and N. G. Greenet al., J. Am. Phys. Soc., 2000, Physical Review E, 61, 4011-4018 and4019-4028.

At a certain threshold flow velocity, at which positivedielectrophoretic forces are overcome by drag forces, the bulk flowmoves the particles onto the electrode surfaces. The particles collectalong the surface of the electrodes at the point where the bulk flowdiminishes or converges with a bulk flow originating from an opposingedge of the same electrode. The particles are apparently held on thesurface of the electrode by one or more of a positive dielectrophoreticforce, flow stagnation and gravity.

The present invention starts from the realisation that the applicationof an electric field at or adjacent a sensor surface may offer increasedsensitivity for the detection and/or assay of particles. In particular,abnormal dielectrophoresis may be particularly suitable in thatparticles are collected on an upper surface of an electrode.

The present invention generally aims to provide an improved electrodeconfiguration capable of focusing particles on an electrode surfacethereof. The present invention, also aims to provide surface boundsensors associated with or incorporating an electrode configurationwhich is capable of collecting or focusing particles from a liquid ontoan electrode surface.

Accordingly, in one aspect, the present invention provides apparatus forcollecting particles, comprising a first planar electrode having anupper surface defining one or more substantially circular areasseparated from a planar counter electrode having an upper surfacebounding, at least in part, the circular area or areas of the firstelectrode by a predetermined gap and means for applying an alternatingpotential difference of a predetermined magnitude, at a predeterminedfrequency lower than the charge relaxation frequency of the liquid,across the electrodes whereby particles from a liquid are focused ontothe circular area or areas of the first electrode through an inducedelectro-osmotic flow in the liquid.

It will be understood that the term “substantially circular” does notstrictly require that the area or areas, which it qualifies, are whollyor truly circular. In particular, it will be apparent that thecontribution of the present invention is to provide for the collectionof particles by convergence of bulk flows in the liquid originating ator adjacent the bounded edges of an electrode. Thus, the term“substantially circular” may refer to whole or partially interruptedcircles as well as to distorted circles.

In preferred embodiments of the present invention, the counter electrodecomprises a surface bounding at least the most part of the circular areaor areas of the first electrode.

In some embodiments, the upper surface of the first electrode is whollycircular and the upper surface of the counter electrode provides awholly unbroken circular recess there around. In this embodiment thecounter electrode can comprise a ring electrode surrounding the firstelectrode. However, these embodiments are difficult to fabricate in thatthe first electrode can only be connected to an alternating signalsource by covering a portion of the counter electrode with an insulatingmaterial.

In another embodiment, therefore, the upper surface of the firstelectrode defines a circular area having a leg portion and the counterelectrode comprises a broken ring surrounding the major part of thecircular area of the first electrode. In this “lollipop” embodiment, theinterruption in the ring bounds a portion of the leg portion of thefirst electrode surface.

Preferably, however, the electrode configuration is interdigital. Insome embodiments, the upper surface of the first electrode comprises aplurality of circular areas having a common, horizontal central axisconnected by connecting arm portions. The arms may lie on oralternatively be offset from the common axis of the circular areas. Inthese “pearl chain” embodiments, the counter electrode may comprise aplurality of recesses and “leg” or “tooth” portions bounding neighboringcircular areas of the upper surface of the first electrode. Preferably,the leg or teeth portions define opposing, arcuate edges so as toincrease the circular areas bound by the counter electrode.

In a particularly preferred embodiment of the present invention, theupper surface of the first electrode defines a plurality of circularareas, sharing a common horizontal central axis, each having aconnecting limb, substantially perpendicular, to the main body or “busbar” of the electrode. In this embodiment, the common axes of the firstelectrode and the counter electrode are arranged parallel to one anotherwith the limbs and bus bar of each electrode bounding the most part ofeach circular area of the other electrode.

It will be understood that the latter configuration is particularlyadvantageous in that particles are also focused on the circular areas ofboth the first electrode and the counter electrode. The configurationtherefore provides optimal use of the electrode surfaces for focussingparticles and is easily fabricated in a single step.

The circular area or areas of the electrode configurations of thepresent invention, may comprise a wide range of diameters. Inparticular, the diameter of the, or each circular area may be betweenone and three orders of magnitude higher, than the size of the particlesof interest. By contrast, conventional electrode configurationssupporting abnormal dielectrophoresis (see for example N. G. Green andH. Morgan, J. Phys., D, Appl. Phys., 1998, 31, L25-L30) employ surfaceshaving diameters which are limited to a single order of magnitudegreater than particle size. The present invention is not restricted bythis apparent requirement, which is thought necessary to maintain anappropriate electric field gradient. Without wishing to be bound bytheory, the collection or focusing of particles would appear to dependlargely on the generation of the bulk flow in the liquid.

The bulk flow induced in the liquid comprises vortices developing at theedges of the circular areas and extending over their surfaces. The sizeof each vortex is independent of the size of the electrode butdependant, for a given conductivity of the liquid, on the magnitude andfrequency of the applied alternating potential difference applied to theelectrodes. The size of each vortex peaks at a certain frequency of theapplied potential difference.

The bulk flow in the liquid is also influenced by the extent of the gapbetween the first electrode and the counter electrode. In particular,the electro-osmotic flow, which is dependent on the on the strength ofthe electric field, is inversely proportional to the extent of the gapbetween the electrodes.

The diameter of the electrode surfaces and the extent of the gap betweenthem are chosen so as to enable large vortex sizes to operate in theliquid without overlap on the electrode surfaces. Preferably, thediameter of the electrode surfaces is relatively large compared to theprior art. Still more preferably, the diameter of each circular arearanges from 200 to 1000 μm. Preferably, the extent of the gap rangesfrom 10 to 200 μm. Still more preferably, the extent of the gap rangesfrom 75 to 100 μm.

The choice of frequency and magnitude of the applied potentialdifference for a particular electrode configuration, particle size andliquid conductivity is a matter of routine experimentation. Thefrequency and magnitude of the applied potential difference are chosenso that the vortices developed in liquid do not overlap on the electrodesurfaces. Preferably, the frequency, which may be higher or lower thanthe frequency at which the size of the vortices is greatest, is chosenso that the diameter of the vortices are just below the radius of thecircular area and particles are focused to a tight spot. However,frequencies in which the diameter of the vortices is smaller andparticles are collected in a ring may be used.

It will be appreciated that the electrode configuration of the presentinvention enables relatively large vortices to “reach out” into theliquid to “pull down” the particles to the electrode surfaces. Thepresent invention offers improved sensitivity in detection of theparticles not just through focusing of particles but also through theconcentration effect of relatively large vortices.

The electrodes of the present invention may comprise any suitablyconducting material. Preferably, the thickness of the conductingmaterial ranges from 10 nm to 50 μm. The electrodes may be disposed onany convenient substrate comprising an insulating material. Preferably,the substrate comprises a glass or an optically transparent material.The apparatus of the present invention may, therefore, comprise a chip,which can be conveniently fabricated by photolithography and wet etchinga layer of indium tin oxide (ITO) or gold on a glass substrate.

The apparatus of the present invention may comprise an array of one ormore electrode configurations, comprising any number of circular areas,capable of collecting or focusing particles on a surface of anelectrode.

The present invention allows focusing of particles having a wide rangeof diameters on a single electrode configuration or array. For example,electrode configurations defining surfaces having circular diametersranging from 200 to 1000 μm, and a gap ranging from 10 to 200 μm arecapable of tightly focusing cells, bacteria and viruses from an aqueoussample.

In particular, particles of diameter ranging from 20 nm to 5 μmsuspended in aqueous liquids ranging in conductivity from 10 to 90 mS/mmay be focused by applying a potential difference ranging in frequencyfrom 0.6 to 2.5 kHz and voltage from 1 to 20 V. However, frequenciesranging from 200 to 10,000 Hz may be used depending on the conductivityof the liquid.

It will be understood from the above, that the apparatus of the presentinvention may comprise a surface plasmon resonance (SPR) chip enablingparticles to be detected by a change in the SPR angle of light incidentthe chip and/or by scattering or emission of light from the particles.

In a preferred embodiment, the chip includes a sensing medium capable ofretaining the collected or focused particles. The sensing medium maycomprise a coating or layer of one or more molecules capable ofselectively binding particles of interest—such as antibodies or lectinmolecules.

In these embodiments, the apparatus may comprise a metal-clad (ordye-clad) leaky waveguide (MCLW) sensor such as those described in ourco-pending international patent application PCT/GB2002/04545.

Further, the antibody layer, for example, may be provided over one ormore circular areas of the electrode. Of course, different antibodylayers may be provided over different circular areas of the electrodeor, where more than one electrode pair is used, over differentelectrodes.

In a second aspect, the present invention provides a method forcollecting particles from a liquid, comprising the steps of i)introducing the liquid to apparatus comprising a first electrode, havingan upper surface defining one or more substantially circular areas ofpredetermined diameter, separated from a counter electrode, having anupper surface bounding at least in part the circular area or areas ofthe first electrode, by a predetermined gap and ii) applying analternating potential difference of a predetermined magnitude at apredetermined frequency lower than the charge relaxation frequency ofthe liquid whereby to induce an electro-osmotic flow in the liquid thatfocuses the particles onto the circular area or areas of the firstelectrode.

In one embodiment of the method, the magnitude and frequency of thepredetermined alternating potential is chosen, for a particularelectrode configuration, particle size and liquid conductivity, so thatthe electro-osmotic flow defines vortices of size just below the radiusof the circular area or areas of the first electrode. In this embodimentthe focusing of the particles is tightly confined to the centre of eachfocusing surface.

As mentioned above, preferably the magnitude and frequency of thepredetermined alternating potential is also chosen, for a particularelectrode configuration and liquid conductivity, so that the vortices“reach out” into the bulk of the liquid to “pull down” the maximumpossible number of particles.

For liquids of conductivity ranging from 10 to 90 mS/m, the appliedpotential difference preferably ranges in frequency from 0.6 to 2.5 kHzand voltage from 1 to 20 V when the diameter of the circular area orareas ranges from 200 to 1000 μm and the gap between electrodes rangesfrom 10 to 200 μm. However, as mentioned above, frequencies ranging from200 to 10,000 Hz may be used depending on the conductivity of theliquid.

In one embodiment of the method, the frequency and magnitude of theapplied potential difference is chosen, for a particular electrodeconfiguration and liquid conductivity, so that viruses are tightlyfocused. In another embodiment, the magnitude and frequency of theapplied potential difference is chosen, for a particular electrodeconfiguration and liquid conductivity, so that bacteria are tightlyfocused. It will be apparent therefore that the present inventionenables the focused collection of a wide range of particles. Inparticular, it is expected that particles ranging in diameter from 20 nmto 5 μm may be focused.

Where the apparatus of the present invention comprises an SPR or MCLWchip, the method of the present invention enables the detection and/orassay of particles in the liquid. As mentioned above, the particles maybe detected by a change in the SPR angle of light incident the chipand/or by scattering or emission of light from the particles.

In a preferred embodiment, the method uses a chip incorporating asensing medium, such as an antibody or lectin coating or layer, capableof retaining the collected or focussed particles. In this embodiment,the particles may be retained when the electrode power or signal sourceis turned off.

The method of the present invention may be adapted to remove (or “wash”)particles from an electrode surface on which they are collected orbound, for example, by antibody coating or layer. In particular, thefrequency of the signal source is chosen so that the vortices in thefluid overlap the electrode surface.

In one embodiment, particles not retained over the electrode surface bybonding with, for example, an antibody or lectin, are washed away by thefluid. The particles of interest may be separated from, for example,unwanted particles such as dust.

The association of a surface bound sensor with an electrodeconfiguration providing an electric field offers the advantage ofincreased sensitivity in the detection and assay of particles in aliquid. In particular, apparatus including an electrode or electrodessupporting abnormal dielectrophoresis are less sensitive to theparticular properties of the particles in that they principally rely onbulk flow in the liquid rather than their interaction with an electricfield. Thus, a single apparatus may be used for the collection anddetection of a wide range of particles including viruses, bacteria andcells.

The incorporation of the electrode or electrodes into the surface of thesensor enables easy fabrication and the relatively low frequencies usedcan be generated by standard signal generators and amplifier chipsoffering low power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by reference to a number ofexamples and the following drawings in which

FIGS. 1 a) and b) are schematic illustrations highlighting thecollection of particles from a liquid onto an electrode surface throughabnormal dielectrophoresis;

FIGS. 2 a) to c) are plan views of a number of different embodiments ofthe electrode configuration according to the present invention;

FIG. 3 is a plan view of a preferred embodiment of the electrodeconfiguration according to the present invention;

FIGS. 4 a) to c) are photographs highlighting the collection of bacillussubtilis var. Niger (bacillus globigii) spores on the electrode surfacesof the embodiment of FIG. 3;

FIGS. 5 a) and b) are graphs showing respectively the collection ofparticles on the electrode surfaces of the embodiment of FIG. 3 as afunction of time and as a function of the frequency of the alternatingapplied potential difference;

FIGS. 6 is a photograph showing the collection of virus sized,fluorescent latex beads on the electrode surfaces of the embodiment ofFIG. 3;

FIG. 7 shows focusing of yeast cells;

FIG. 8 is a schematic illustration highlighting the washing of particlesfrom an electrode surface according to one aspect of the presentinvention; and

FIGS. 9 a) and b) are schematic illustrations showing respectively a SPRchip according to the present invention and its use with a suitable flowcell.

Having regard now to FIG. 1 a), there is shown a known electrodeconfiguration, generally designated 11, having first and second planarelectrodes 12, 13 defining a gap 14 between them. An alternatingpotential difference applied across the electrodes 12, 13 at lowfrequency (<100 kHz) leads to an inhomogeneous electric field thatexerts “electro-osmotic” forces on a liquid in contact with theelectrodes.

The forces acting on the liquid generate a bulk flow or vortices 15 inthe liquid which, provided the frequency of the applied potentialdifference is lower than the charge relaxation frequency of the liquid,originates in the gap 14 and moves out across each electrode surface 12a, 13 a.

The bulk flow 15 creates drag forces, which act on particles 16suspended in the liquid. At a critical point, depending on the magnitudeand frequency of the applied potential difference, their magnitude issufficient to overcome dielectrophoretic forces so that particles 16entrained in the bulk flow in the gap are moved from the edges outacross the surface 12 a 13 a of the electrodes. However, because theelectric field is more uniform over the electrode surfaces 12 a 13 a andthe bulk flow 15 diminishes, the upward drag acting on the particles isnow insufficient to overcome the dielectrophoretic forces and/orgravity.

FIG. 1 b) shows part of a known electrode configuration, generallydesignated 17, in which a surface 12 a of a first planar electrode 12 isbounded at opposing edges by surfaces of an adjacent counter electrode18. As may be seen, abnormal dielectrophoresis leads to vortices 15covering substantially the whole of the surface 12 a of the firstelectrode 12 so that the leakage of particles 16 away from the surfaceis effectively contained. The particles 16 are collected on theelectrode surface 12 a in a line parallel to the longitudinal length ofthe electrode 12.

Referring now to FIG. 2 a), there is shown a first, practical embodimentof an electrode configuration according to the present invention. Theelectrode configuration, generally designated 19, comprises a first,substantially circular, planar electrode 20 having a leg 21 separatedfrom a counter electrode. The counter electrode comprises an interruptedring electrode 22, which substantially bounds the first electrode 20 anda portion of the leg 21. Because the first electrode 20 is near circularand bounded around most of its circumference particles 16 collectthereon in a ring, which can be tightly focused to a spot 23 byselection of appropriate parameters. The focusing of particles 16 is dueto a plurality of inwardly extending vortices 15 forming around thecirumferential edge of the electrode 20.

FIG. 2 b) shows an interdigital electrode configuration generallydesignated 24. A first planar electrode 25 comprises four near circularsurfaces 26 linked along a central horizontal axis by arm portions 27. Aplanar, counter electrode 28 defines a surface 28 a having a pair oftoothed leg portions 29 and recesses 30 that complement the circularsurfaces 26 and encompass the first electrode 25. Particles 16 arefocused in a ring or spot 23 on each of the circular surfaces 26 of thefirst electrode 25.

FIG. 2 c) shows a further interdigital electrode configuration. A firstplanar electrode 31 comprises a linear strip or “bus bar” 32 havingthree protruding near circular surfaces 33, which share a common centralhorizontal axis. A planar counter electrode 34 defines a surface 34 ahaving four leg portions 35 and recesses 36 that complement the circularsurfaces and encompass the first electrode 31. Particles 16 are focusedin a ring or spot 23 on each of the circular surfaces 33 of the firstelectrode 31.

FIG. 3 shows another interdigital electrode configuration, generallydesignated 37, which makes optimal use of the electrode surfaces forcollecting or focusing particles 16. Each electrode 38 39 comprises astrip or bus bar 40 having four thumb sections 41 extendingperpendicular to its longitudinal length. Each thumb section 41 definesa near circular surface 42 linked to the strip by an arm 43, andtogether with the bus bar 40 provides complementary recesses 44 for thethumb sections 41 of the other electrode. Particles 16 are focused in aring or spot 23 on the circular surfaces 42 of each electrode 38, 39.

EXAMPLE 1

The electrode configuration 37 of FIG. 3, in which the circular surfaces42 of each electrode 38, 39 are 575 μm and the gap size is 100 μm, isused to focus spores 16 of bacillus globiggi (˜800 nm) from a 30 mS/mpotassium chloride solution containing ˜10⁸ spores/ml at an appliedalternating potential difference of 1 kHz frequency at 10V.

FIG. 4 a) shows the focusing of the spores 16 on a single circularsurface 42 at various time intervals. As may be seen, in the absence ofthe potential difference the spores 16 are randomly distributed in thesolution. After the application of the potential difference for 90seconds, a large number of spores 16 are focused in the centre of thesurface 42. After a further 90 seconds, the spores are tightly focusedin a spot 23 in the centre of the surface 42.

The speed of focusing is dependent on the frequency of the appliedpotential difference and the conductivity of the liquid. Table 1 showsoptimal frequencies for focusing the spores 16 from potassium chloridesolutions of differing conductivities. A decrease in the speed offocusing is observed in solutions of high conductivity which may,however, be compensated by higher voltages.

TABLE 1 Conductivity/mSm⁻¹ Optimal frequency/Hz 10 600 30 1000 70 200090 2500

EXAMPLE 2

The electrode configuration 37 of FIG. 3, in which the circular surfaces42 of each electrode 38, 39 are 575 μm and the gap size is 100 μm, isused to focus spores 16 of bacillus globiggi (˜800 nm) from a 30 mS/mpotassium chloride solution containing ˜10⁵ spores/ml at an appliedalternating potential difference of 1 kHz frequency at 10V.

The number of spores 16 observed at 15 second intervals in the centre ofa circular surface is examined under a high magnification microscope atvarious time intervals.

FIG. 4 b) shows that in the absence of the applied potential differencesixteen spores 16 are present. After the application of the potentialdifference for 180 seconds the number is 113 and rises to 215 after 405seconds. FIG. 5 a) shows the rise in spore number plotted against time.

The electrode configuration of FIG. 3 clearly increases the localconcentration of spores by collecting them on the electrode surface 42.It is expected that optimisation of parameters will lead to a more rapidfocusing of spores from suspensions of this concentration.

As mentioned above, optimum focusing of particles is achieved when thesize of the vortices are limited to just below the radius of thecircular areas of the electrode so that there is no overlap of vorticesoriginating at opposite points on the circumferential edge.

FIG. 4 c) shows the effect of the frequency of the applied potentialdifference on the size of the vortices formed in the liquid and thus thefocusing of the particles on the circular surfaces 42. The collectionfronts 45, 46 observed on the bus bar and on the surfaces 42 provides acontrol means allowing certainty that the diameter of the surfaces 42are appropriate to the frequency of the applied potential difference.However, if the diameter is much larger than the vortices, particles 16will collect on the surface in a ring around the edge. If the diameteris smaller than the vortices, the vortices will overlap exerting anincreased drag on the particles 16 which will no longer be retained onthe surface 42 (see also FIG. 8).

EXAMPLE 3

The electrode configuration 37 of FIG. 3, in which the circular surfaces42 of each electrode 38, 39 are 230 μm and the gap size is 55 μm, isused to focus spores of bacillus globigii (˜800 μm) from a 10 mS/mpotassium chloride solution at an applied alternating potentialdifference of varying frequency at 10V.

FIG. 5 b) shows a plot of the distance of the collection front from theedge of the circle as a function of frequency. As may be seen at thevortices are larger at lower frequencies.

EXAMPLE 4

The electrode configuration of FIG. 3, in which the circular surfaces 42of each electrode 38, 39 are 575 μm and the gap size is 100 μm, is usedto focus fluorescent latex beads of 110 nm diameter from a 1 mS/mpotassium chloride solution at an applied alternating potentialdifference of 10V at 600 Hz frequency.

Referring now to FIG. 6, in the absence of the applied potentialdifference a transmitted light photograph is not instructive. However,after 60 seconds and 120 seconds, fluorescence from the beads clearlyshows them concentrated in the middle of the surfaces 42. The area atthe top of the bus bar shows only background fluorescence of randomlydistributed particles.

The concentration of virus size particles is an encouraging result inview of the fact that viruses may be smaller than other particles andtherefore more likely to recirculated with the bulk flow. However, anantibody coating or layer capable of binding viruses would mitigate thiseffect.

EXAMPLE 5

The electrode configuration 37 of FIG. 3, in which the circular surfaces42 of each electrode 38, 39 are 575 μm and the gap size is 100 μm, isused to focus yeast cells (˜5 μm) from a 0.1 mS/m potassium chloridesolution at an applied alternating potential difference of frequency 600Hz at 10V. Referring now to FIG. 7, after 45 seconds yeast cells 16 aredrawn inwards onto the circular surfaces 42 in the form of a ring. After90 seconds the cells are focused in a tight spot in the centre of thecircular surfaces 42.

Referring now to FIG. 8, the washing of particles over an electrodesurface 47 on which particles are retained by an antibody or lectincoating or layer involves the selection parameters so that vortices 15overlap above the electrode surface 47. As mentioned above, the overlapleads to increased drag on the particles 16, which returns unboundparticles 16 a to the bulk flow.

Referring now to FIGS. 9 a) and b), an electrode configuration similarto that described in FIG. 3 is provided in the centre of a circular SPRchip. The electrodes 38 39, which are gold, comprise semi-circular busbars 40, which are connected to an alternating signal source (not shown)via a spring 48.

The arrangement may also be provided with semi-circular touch pads (notshown) so as to allow the slide to be rotated for different flowdirections. The chip is fitted to a flow cell 49 provided with a lid 50defining a central aperture 51 through which the spring 48, secured by asealant glue 52, extends.

1. Apparatus for collecting particles from a liquid, comprising a firstelectrode having an upper surface defining one or more substantiallycircular areas of a predetermined diameter separated from a counterelectrode, having an upper surface bounding at least in part thecircular area or areas of the first electrode, by a predetermined gapand means for applying an alternating potential difference of apredetermined magnitude at a predetermined frequency lower than thecharge relaxation frequency of the liquid across the electrodes wherebyparticles from the liquid are focused onto the circular area or areas ofthe first electrode through an induced electro-osmotic flow in theliquid.
 2. Apparatus according to claim 1, in which the upper surface ofthe counter electrode defines one or more substantially circular areasof predetermined diameter.
 3. Apparatus according to claim 1, in whichthe electrode configuration is interdigital.
 4. Apparatus according toclaim 3, in which the upper surface of the first electrode defines aplurality of substantially circular areas sharing a first commonhorizontal axis.
 5. Apparatus according to claim 4, in which the uppersurface of the counter electrode defines a plurality of substantiallycircular areas sharing a second horizontal axis.
 6. Apparatus accordingto claim 5, in which the first and second horizontal axis are paralleland the upper surface of the each electrode bounds at least a portion ofthe a circular area of the other electrode.
 7. Apparatus according toclaim 1, in which the predetermined diameter of the, or each circulararea ranges from 200 to 1000 μm and the extent of the predetermined gapranges from 10 to 200 μm.
 8. Apparatus according to claim 1, in whichthe first and second electrodes comprise a metal capable of supportingsurface plasmon resonance.
 9. Apparatus according to claim 1, in whichone or more circular areas on the first or each electrode are coatedwith an antibody or lectin layer.
 10. Apparatus according to claim 1, inwhich the electrodes are disposed on glass, or an optically transparent,substrate.
 11. Apparatus according to claim 10, in which the substrateis part of an SPR or MCLW sensor.
 12. Apparatus according to claim 1,comprising an array of electrodes.
 13. A method for collecting particlesfrom a liquid, comprising the steps of i) introducing the liquid to afirst electrode having an upper surface with one or more substantiallycircular areas of predetermined diameter separated from a counterelectrode having an upper surface bounding at least in part the circulararea or areas of the first electrode, by a predetermined gap and ii)applying an alternating potential difference of a predeterminedmagnitude at a predetermined frequency lower than the charge relaxationfrequency of the liquid across the array whereby to induce anelectro-osmotic flow in the liquid that focuses the particles from theliquid onto the circular area or areas of the first electrode.
 14. Amethod according to claim 13, in which the frequency of thepredetermined alternating potential induces an electro-osmotic flowextending from an edge of the first electrode substantially across thewidth of the radius of the or each circular area.
 15. A method accordingto claim 13, in which the conductivity of the liquid ranges from 10 to90 mS/m.
 16. A method according to claim 13, in which the frequency ofthe applied potential difference ranges from 0.6 to 2.5 kHz at a voltageranging from 1 to 50V.